1. Field of the Invention
The present invention relates to compass systems and methods, but more particularly, it relates to a nonmagnetic and noninertial compass system and method incorporating a global positioning system (GPS), such as the NAVSTAR/GPS multisatellite system.
2. Description of the Prior Art
Current navigation systems which yield compass heading u information relay on magnetic compasses, gyrocompasses, and other inertial navigation systems. Magnetic compasses are unreliable in the Northern latitudes and require knowledge of position in order to compensate for magnetic variation. Gyrocompasses lose accuracy as the square of the time elapsed since their initial zeroing. This initial calibration also requires a knowledge of a heading for the platform. Consequently, there is a need in the prior art to acquire accurate compass heading information without being affected by magnetic anomalies, and without being dependent on the elapsed time after an initial zeroing or calibration of the compass and/or knowledge of the initial heading of the platform.
In the near future, it is believed that most commercial and military ships and aircraft will make use of the NAVSTAR/GPS multi-satellite system in order to determine position anywhere in the world. The contemplated 18 to 24 satellites will generate precisely timed radio signals which will enable ground, sea, or airborne GPS receivers to determine their absolute self-position within 10 meters (for instantaneous measurements) or to within tenths of a meter when special algorithms and time integration techniques are used. At the present time, the NAVSTAR/GPS multi-satellite system is operational with six satellites in earth orbit. With position data from four satellites, an advanced receiver, such as the Texas instrument 4100 or the Litton Industries LTN-700, can compute position in three dimensions, as well as velocity With position data from three satellites, a three-dimensional position fix can be obtained, i.e., longitude, latitude and altitude. However, if altitude and time are known, position data from only two satellites are required to obtain a two-dimensional position fix, i.e., longitude and latitude.
There are several established operational modes for the NAVSTAR/GPS multi-satellite system. Of particular interest, as related to the present invention, is the standard navigation mode, where one antenna and one GPS receiver are used for either shipboard or airborne navigation, and the differential positioning mode, where two antennas and their corresponding two GPS receivers are linked to provide a measurement of the difference in position between the GPS receivers. The differential positioning mode has application for surveying and positioning where one of the GPS receivers is placed at an established benchmark. The other GPS receiver might, for example, be placed aboard an oil rig that is being moved particular location. The advantage of this differential mode is its extreme accuracy. This is because navigation errors problems with transmission of the satellite signals through the atmosphere, e.g., the ephemeris errors, are cancelled. Thus, computation of the relative position of two GPS receivers can be significantly more accurate than an absolute position measurement using one GPS receiver. Consequently, there is a need in the prior art to take advantage of the extreme accuracy of the differential mode in the configuration of nonmagnetic and noninertial compass systems and methods (a compass mode), while eliminating the requirement for two GPS receivers.
The prior art, as indicated hereinabove, includes advances in compass systems and methods incorporating the NAVSTAR/GPS multi-satellite system. However, insofar as can be determined, no prior art compass system or method incorporates all of the features and advantages of the present invention.